1. Field of the Invention
The present invention relates generally to color cathode ray tube (CRT) display apparatus and more particularly to CRT displays used in applications under which the ambient light conditions vary over a very wide range. One such application is an aircraft cockpit wherein the ambient light can vary from direct, high altitude sunlight to almost total darkness. High contrast enhancement filter techniques of the type disclosed in the present assignee's U.S. Pat. No. 3,946,267 are used to maintain the desired contrast ratios under such light ambients. More specifically, the present invention relates to CRT display apparatus; for example a shadow-mask type color CRT, for use in such ambient light conditions which automatically and independently adjusts the cathode drive voltage of the cathode for each of the color phosphors dependent upon each of the phosphor's light emissive characteristic at a variable reference brightness and in accordance with the display writing technique being used, i.e., raster or stroke. In addition, the apparatus of the invention may include a provision for providing a reference focus of the cathode beam for ecah color in accordance with the reference brightness.
2. Description of the Prior Art
In most prior art CRT display systems, such as for example, home and commercial TV's, where normal viewing ambient light conditions do not vary significantly or where if viewing is in high ambient light conditions mechanical shadesor baffles are used to prevent direct sunlight from impinging upon the CRT face, essentially fixed predetermined drive voltages for the green, red and blue cathodes are used. Thus, any changes in the manual brightness setting causes only a d.c. shift in the voltages applied to the CRT. To restore the proper colors, readjustment of the green, red, and blue guns is necessary. Since the adjustments are over a relatively narrow range of ambient light conditions, the color shift is slight and generally ignored. The automatic brightness function on commercial TV's affects the drive of all three guns in identically the same manner and has no features to compensate for color shifts; but again the small operating envelope keeps the error from being objectionable.
Thus, known conventional color CRT brightness controls, whether automatic, manual or both are unsuitable for use in color CRT's used to display information in an aircraft cockpit environment.